The present invention concerns methods and arrangements for the optical measurement of the concentration of a substance of interest by resort to light measurement utilizing a monochromator and a light-metering unit. With methods and arrangements of this type, an indicator is contained in an indicator chamber at least a part of which is bounded by a membrane permeable to the substance of interest, with at least a part of the indicator chamber furthermore being transmissive for radiation entering and exiting the indicator chamber. The permeable membrane is placed in contact with the substance of interest, and molecules or particles of the substance of interest diffuse into the indicator chamber. Radiation from the monochromator is directed into the indicator chamber, and the spectral response of the indicator therein to the monochromator radiation (i.e., a change of color or a fluorescent response) depends upon the concentration of the substance of interest.
Using arrangements and methods of the type in question, it is necessary to calibrate the indicators employed, when high-precision concentration measurements are called for. This may be quite difficult, especially inasmuch as the indicator within the indicator chamber is generally not accessible but instead blocked from access by the aforementioned permeable membrane.
The indicator-chamber structure, containing an indicator and bounded by the aforementioned permeable membrane and radiation-transmissive wall is referred to herein as an optode, a term used in previous patents involving such type of indicator-chamber structure, e.g., U.S. Pat. No. 4,003,707.
One way to calibrate such an optode is by performing a concentration measurement with respect to a comparison substance whose concentration is known in advance, e.g., using a comparison sample containing the substance of interest in a known concentration. However, there exist applications in which this would be problematic due to the remoteness of the optode from the sample of interest, for example as in the case of blood gas concentration measurements performed using a catheter inserted into a blood vessel.